The interaction between the immune system and the Gross passage A leukemic virus (GPA) is under study in an attempt to outline the sequence of dissemination of the malignancy in neonate, weanling, and adult mice. It was discovered that preleukemic skin grafting transmits GPA lymphoma horizontally to weanling and adult animals despite the immunologic competence of the adult animals. Other preleukemic organ implants and suspensions are being tested for their effectiveness in horizontal transmission. In addition, several facets of immune responsiveness in leukemogenesis will be assessed: thymus-peripheral node interaction, germinal center response, establishment of immunization protocols, and heightening immune response to established tumors.